1. Technical Field
This invention is generally related to nucleic acid sequencing, as well as methods and products relating to the same.
2. Description of the Related Art
Nucleic acid sequences encode the necessary information for living things to function and reproduce, and are essentially a blueprint for life. Determining such sequences is therefore a tool useful in pure research into how and where organisms live, as well as in applied sciences such as drug development. In medicine, sequencing tools can be used for diagnosis and to develop treatments for a variety of pathologies, including cancer, heart disease, autoimmune disorders, multiple sclerosis, or obesity. In industry, sequencing can be used to design improved enzymatic processes or synthetic organisms. In biology, such tools can be used to study the health of ecosystems, for example, and thus have a broad range of utility.
An individual's unique DNA sequence provides valuable information concerning their susceptibility to certain diseases. The sequence will provide patients with the opportunity to screen for early detection and to receive preventative treatment. Furthermore, given a patient's individual blueprint, clinicians will be capable of administering personalized therapy to maximize drug efficacy and to minimize the risk of an adverse drug response. Similarly, determining the blueprint of pathogenic organisms can lead to new treatments for infectious diseases and more robust pathogen surveillance. Whole genome DNA sequencing will provide the foundation for modern medicine.
DNA sequencing is the process of determining the order of the chemical constituents of a given DNA polymer. These chemical constituents, which are called nucleotides, exist in DNA in four common forms: deoxyadenosine (A), deoxyguanosine (G), deoxycytidine (C), and deoxythymidine (T). Sequencing of a diploid human genome requires determining the sequential order of approximately 6 billion nucleotides.
Currently, most DNA sequencing is performed using the chain termination method developed by Frederick Sanger. This technique, termed Sanger Sequencing, uses sequence specific termination of DNA synthesis and fluorescently modified nucleotide reporter substrates to derive sequence information. This method sequences a target nucleic acid strand, or read length, of up to 1000 bases long by using a modified polymerase chain reaction. In this modified reaction the sequencing is randomly interrupted at select base types (A, C, G or T) and the lengths of the interrupted sequences are determined by capillary gel electrophoresis. The length then determines what base type is located at that length. Many overlapping read lengths are produced and their sequences are overlaid using data processing to determine the most reliable fit of the data. This process of producing read lengths of sequence is very laborious and expensive and is now being superseded by new methods that have higher efficiency.
The Sanger method was used to provide most of the sequence data in the Humane Genome Project which generated the first complete sequence of the human genome. This project took over 10 years and nearly $3B to complete. Given these significant throughput and cost limitations, it is clear that DNA sequencing technologies will need to improve drastically in order to achieve the stated goals put forth by the scientific community. To that end, a number of second generation technologies, which far exceed the throughput and cost per base limitations of Sanger sequencing, are gaining an increasing share of the sequencing market. Still, these “sequencing by synthesis” methods fall short of achieving the throughput, cost, and quality targets required by markets such as whole genome sequencing for personalized medicine.
For example, 454 Life Sciences is producing instruments (e.g., the Genome Sequencer) that can process 100 million bases in 7.5 hours with an average read length of 200 nucleotides. Their approach uses a variation of Polymerase Chain Reaction (“PCR”) to produce a homogeneous colony of target nucleic acid, hundreds of bases in length, on the surface of a bead. This process is termed emulsion PCR. Hundreds of thousands of such beads are then arranged on a “picotiter plate”. The plate is then prepared for an additional sequencing whereby each nucleic acid base type is sequentially washed over the plate. Beads with target that incorporate the base produce a pyrophosphate byproduct that can be used to catalyze a light producing reaction that is then detected with a camera.
Illumina Inc. has a similar process that uses reversibly terminating nucleotides and fluorescent labels to perform nucleic acid sequencing. The average read length for Illumina's 1G Analyzer is less than 40 nucleotides. Instead of using emulsion PCR to amplify sequence targets, Illumina has an approach for amplifying PCR colonies on an array surface. Both the 454 and Illumina approaches use a complicating polymerase amplification to increase signal strength, perform base measurements during the rate limiting sequence extension cycle, and have limited read lengths because of incorporation errors that degrade the measurement signal to noise proportionally to the read length.
Applied Biosystems uses reversible terminating ligation rather than sequencing-by-synthesis to read the DNA. Like 454's Genome Sequencer, the technology uses bead-based emulsion PCR to amplify the sample. Since the majority of the beads do not carry PCR products, the researchers next use an enrichment step to select beads coated with DNA. The biotin-coated beads are spread and immobilized on a glass slide array covered with streptavidin. The immobilized beads are then run through a process of 8-mer probe hybridization (each labeled with four different fluorescent dyes), ligation, and cleavage (between the 5th and 6th bases to create a site for the next round of ligation). Each probe interrogates two bases, at positions 4 and 5 using a 2-base encoding system, which is recorded by a camera. Similar to Illumina's approach, the average read length for Applied Biosystems' SOLiD platform is less than 40 nucleotides.
Other approaches are being developed to avoid the time and expense of the polymerase amplification step by measuring single molecules of DNA directly. Visigen Biotechnologies, Inc. is measuring fluorescently labeled bases as they are sequenced by incorporating a second fluorophore into an engineered DNA polymerase and using Forster Resonance Energy Transfer (FRET) for nucleotide identification. This technique is faced with the challenges of separating the signals of bases that are separated by less than a nanometer and by a polymerase incorporation action that will have very large statistical variation.
A process being developed by LingVitae sequences cDNA inserted into immobilized plasmid vectors. The process uses a Class IIS restriction enzyme to cleave the target nucleic acid and ligate an oligomer into the target. Typically, one or two nucleotides in the terminal 5′ or 3′ overhang generated by the restriction enzyme determine which of a library of oligomers in the ligation mix will be added to the sticky, cut end of the target. Each oligomer contains “signal” sequences that uniquely identify the nucleotide(s) it replaces. The process of cleavage and ligation is then repeated. The new molecule is then sequenced using tags specific for the various oligomers. The product of this process is termed a “Design Polymer” and always consists of a nucleic acid longer than the one it replaces (e.g., a dinucleotide target sequence is replaced by a “magnified” polynucleotide sequence of as many as 100 base pairs). An advantage of this process is that the duplex product strand can be amplified if desired. A disadvantage is that the process is necessarily cyclical and the continuity of the template would be lost if simultaneous multiple restriction cuts were made.
U.S. Pat. No. 7,060,440 to Kless describes a sequencing process that involves incorporating oligomers by polymerization with a polymerase. A modification of the Sanger method, with end-terminated oligomers as substrates, is used to build sequencing ladders by gel electrophoresis or capillary chromatography. While coupling of oligomers by end ligation is well known, the use of a polymerase to couple oligomers in a template-directed process was utilized to new advantage.
Polymerization techniques are expected to grow in power as modified polymerases (and ligases) become available through genetic engineering and bioprospecting, and methods for elimination of exonuclease activity by polymerase modification are already known. For example, Published U.S. Patent Application 2007/0048748 to Williams describes the use of mutant polymerases for incorporating dye-labeled and other modified nucleotides. Substrates for these polymerases also include γ-phosphate labeled nucleotides. Both increased speed of incorporation and reduction in error rate were found with chimeric and mutant polymerases.
In addition, a large effort has been made by both academic and industrial teams to sequence native DNA using non-synthetic methods. For example, Agilent Technologies, Inc. along with university collaborators are developing a single molecule detection method that threads the DNA through a nanopore to make measurements as it passes through. As with Visigen and LingVitae, this method must overcome the problem of efficiently and accurately obtaining distinct signals from individual nucleobases separated by sub-nanometer dimensions, as well as the problem of developing reproducible pore sizes of similar size. As such, direct sequencing of DNA by detection of its constituent parts has yet to be achieved in a high-throughput process due to the small size of the nucleotides in the chain (about 4 Angstroms center-to-center) and the corresponding signal to noise and signal resolution limitations therein. Direct detection is further complicated by the inherent secondary structure of DNA, which does not easily elongate into a perfectly linear polymer.
While significant advances have been made in the field of DNA sequencing, there continues to be a need in the art for new and improved methods. The present invention fulfills these needs and provides further related advantages.